I am looking for a sleeping pad combo to equal an R value of approximately 8. I plan to use a Gossamer Gear Thinlite 1/8" on bottom, an inflatable pad between, and a Gossamer Gear Thinlite 3/8" on top. The question is, what should the intermediate inflatable be? The two thinlite pads give me an R-value of about 1.8. Close enough to 2 for me. So I need a pad that has an R-value of 6 or better. I have been thinking of a POE Hyper Elite at an R-value of 5-14 and 19.5 oz., an Exped Downmat7 pump at R-value 5.9 and 32 oz., or a custom Kookabay synthetic fill mat. I want the CCF in the system for redundancy. I don't want to have a pad failure in the back country at temps. low enough to warrant an R8 rating. Is this paranoid thinking on my part? Any recommendations for or against any of these options? Any other options I've missed? Thanks.

I would recommend trying to get the Rvalue of the CC pads closer to 3 (at least at your torso) and having bender make you a custom DAM to cover the difference. In addition to the gossamer pads I would look at the zlite / ridgerest pads.
This gives you a little margin in case your air mat fails — if you figure you need R-8 I guess it wouldn't hurt to consider said failure.

The DM7 is a great pad. I think you could get a down Kookabay w/similar R-value for nearly 1/2 the weight, might add some extra down just to cover your bases, as it were. I haven't used the Hyper Elite, but it seems like a good option.

It's not really paranoid thinking to have redundancy in this way. But since I started using self-inflaters in the mid-80s or so I've never put a hole in one, nor in my air mats. They travel in my pack & go straight into the tent. Where I've personally found myself using a CCF/inflatable combo is when I've been trying to boost the R-value of a pad… ie a R-4 pad w/a ridge rest gets to ~R-6.

i have on more than one occasion seen inflatable sleeping pads left irreparable by a misplaced crampon. it's one thing to have that happen in the cascades or mt washington where it can be worked around as apposed to some multi-week expedition like denali. there's more than one high altitude specialist that wouldn't include an inflatable pad in their sleeping system for just that reason. that being said there are those that do and with few exceptions the guided trips combine an inflatable with a closed cell foam pad, but not nearly achieving your r-value. i think your system is overkill for a aconcagua.

I don't think you will want to juggle the complexities of three layers of sleeping pad. Normally I use one layer for summer and two layers for winter. For summer on Aconcagua, I used two layers. One was a 3/4 length Thermarest (1") and one was a 3/4 length of cheap CCF. Then I had plenty of thick clothing to augment that if there had been a failure.

One guy almost had his sleeping bag blow away in a gust of wind. That is a human failure that you can't afford.

Personally I wouldn't use inflatables on a long expedition. I've seen them fail on different pads on consecutive trips (one was a slickrock trip ouch). I don't even use a inflatable by itself now, if I'm going with only one I go CCF and have just learned to sleep comfy on it.
If I could I might take an inflatable torso length for comfort.